1. Field of the Invention
The present invention relates generally to a direction finder, and more particularly to a direction finder apparatus and method that detect a RF signal that includes an amplitude modulated signal component.
2. Description of the Background Art
Direction finding is the process of determining the origin of an electromagnetic signal. Direction finding therefore involves detecting the signal and then determining a direction of arrival (DOA) angle to the signal origin point. In order to accurately determine the origin of the broadcast, at least two independent directional readings must be obtained at different locations, referred to as triangulation.
Direction finding has many applications. One application is finding an intended transmission by pinpointing a broadcast origination location. Another common use is for finding a source of interference or noise. As the world becomes more populous and more technologically advanced, there are more and more electronic devices sharing the frequency spectrum. In addition, over time electronic devices deteriorate and may therefore leak noise.
The earliest direction finders were radio receivers, and direction finding consisted of finding a DOA heading by rotating the accompanying antenna (usually of a loop or dish design) until the strongest signal was found. Improvements in direction finder design initially focussed on improved reception and amplification. However, signal differentiation and identification has also been addressed. This is because as the use of the signal spectrum has increased, it has become increasingly costly and complicated to look at ever increasing portions of the signal spectrum. Direction finding was relatively easy when the maximum attainable frequency was below, say, 100 megahertz (MHz). Now, however, when seeking a signal in the gigahertz (GHz) range, a larger portion of the signal spectrum may have to be examined.
In order to refine direction finding and aid in identification of the signal, knowing the approximate characteristics (i.e., the signature) of the signal is helpful. By comparing an unknown signal to known signal signatures, the signal may be more easily identified.
The prior art has used Fourier transforms in order to aid in signal identification and direction finding. The Fourier transform may be used to convert an electromagnetic signal from the time domain to the frequency domain. This may be done to aid in detection and characterization of a subject signal.
FIG. 1 shows an amplitude modulated (AM) radio frequency (RF) electromagnetic signal in the time domain. The time domain signal contains information and is generally in the shape of a sinusoid. Many electronic devices broadcast RF electromagnetic signals, including devices that unintentionally generate and broadcast signals. These unintentional signals may be generated by motors, local oscillators, microwave oven magnetrons, etc. The signal frequency may occupy one or more frequencies, and may vary.
A microwave oven is a good example of an unintended RF signal source. In a microwave oven, the oven generates RF waves that heat food. The RF waves are referred to as microwaves due to their small wavelength. The microwave oven includes a magnetron that generates RF waves of a microwave frequency, generally in the 2,430-2,505 MHZ frequency range (the s-band). The magnetron is triggered by A/C line current cycles, typically 60 hertz (Hz), plus or minus about 20 Hz. The polarity of the magnetron is reversed every time the line current changes. Therefore, a microwave oven magnetron generates a RF pulse train having a substantially regular amplitude modulated waveform of about 60 Hz. As a result, the magnetron produces an unintended amplitude modulated RF signal having a carrier wave of the microwave frequency.
A Fourier transform of the signal of FIG. 1 results in a waveform having a shape corresponding to the presence of sinusoidal frequency components. Complex waveforms may be transformed by the Fourier transformation into sums of simple, sinusoidal functions at different frequencies. Therefore, to determine the response of a certain system to a complex input signal, the input signal may be broken down into a sum of sinusoidal elements and the system response to each sinusoidal element may be analyzed. This technique is referred to as analysis in. the frequency domain and is probably the most widely known and used design and analysis procedure for all types of electrical engineering problems. In general, the Fourier transform is used to move a function from amplitude as a function of time to amplitude as a function of frequency. Looking at a function which describes amplitude in terms of frequency reveals the signal strength in a particular range of frequencies.
A Fourier transform of a pure sinusoid carrier wave would therefore result in essentially an impulse or spike, as there would be no harmonics in the signal. However, when looking for an interfering signal, the bandwidth of a search may be very large and therefore complicated and costly to detect. Moreover, if the signal""s frequency and bandwidth are not known, the direction finder apparatus may have to search a larger than necessary frequency range. In the time domain, a direction finder apparatus would have to apply a filter or bank of filters and measure the filter output(s) in order to detect a signal. This may require a large number of filters in order to cover a desired frequency detection range.
FIG. 2 is a frequency domain representation of a 2.45 GHz signal. In the frequency domain, a detection finder apparatus may more easily detect a signal.
FIG. 3 is a frequency domain representation of a 2.45 GHz signal, showing various possible frequency components (i.e., harmonics). Although the signal contains multiple frequency harmonics, for direction finding the search bandwidth need only extend far enough to detect the main frequency component of the signal (i.e., the main peak). However, it should be noted that this prior art search bandwidth is still quite large. For example, to detect a common microwave oven emission in the 2,430-2,505 MHZ range, the RF search bandwidth would be 2,505-2,430=75 MHZ.
There remains a need, therefore, for improvements in direction finding.
A direction finder apparatus for determining a direction to a device generating a RF signal that includes an amplitude modulated component is provided according to a first embodiment of the invention. The direction finder apparatus comprises an antenna system capable of receiving the signal and a RF front end communicating with the antenna system, the RF front end including at least one filter stage and at least one amplifier stage. The direction finder apparatus further comprises an analog signal detector communicating with the RF front end, the analog signal detector detecting the electromagnetic signal and providing in response a characteristic signal without an underlying carrier wave. The direction finder apparatus further comprises an analog-to-digital (A/D) converter that communicates with the analog signal detector and digitizes the characteristic signal, and a fast Fourier transform (FFT) processor that communicates with the A/D converter. The FFT processor performs a Fourier transform on the digitized signal over a predetermined sampling period. The FFT processor produces a plurality of component frequency approximations representing frequency components of the AM signal. The direction finder apparatus further comprises a plurality of storage bins that communicate with the FFT processor, with the plurality of storage bins storing the plurality of component frequency approximations. The direction finder apparatus further comprises a post-processor that communicates with the plurality of storage bins. The post-processor determines a maximum S/N ratio from among the plurality of component frequency approximations. A DOA angle of the RF signal is determined when the antenna system is oriented to produce a maximum S/N ratio.
A direction finder apparatus for determining a direction to a device generating a RF signal that includes an amplitude modulated component is provided according to a second embodiment of the invention. The direction finder apparatus comprises an antenna means for receiving the RF signal, at least one filtering means for filtering the RF signal, and at least one amplifying means for amplifying the RF signal. The direction finder apparatus further comprises an analog signal detector means for detecting the electromagnetic signal and providing in response a characteristic signal without an underlying carrier wave. The direction finder apparatus further comprises an analog-to-digital (A/D) converter means for digitizing the characteristic signal and a fast Fourier transform (FFT) processing means for performing a Fourier transform on the digitized signal over a predetermined sampling period and producing a plurality of component frequency approximations representing frequency components of the AM signal. The direction finder apparatus further comprises a storage means for storing the plurality of component frequency approximations and a post-processing means for determining a maximum S/N ratio from among the plurality of component frequency approximations. A DOA angle of the RF signal is determined when the antenna means is oriented to produce a maximum S/N ratio.
A method of finding a direction to a RF signal that includes an amplitude modulated signal component is provided according to the invention. The method comprises the steps of detecting the RF signal, detecting the amplitude modulated component, and digitizing the amplitude modulated component. The method further comprises the steps of performing a Fourier transform on the digitized amplitude modulated component to produce a FFT output and determining a direction of arrival angle from a maximum FFT output.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.